1. Field of the Invention
This invention relates to a vehicle antenna which is fixed to an exterior part of a vehicle body of a vehicle, and a diversity receiving apparatus which used the suchlike vehicle antenna.
2. Description of the Related Art
In order to be able to receive broadcasting radio waves and communication radio waves even in case that a vehicle such as an automobile is moving, various vehicle antennas have long been used so far. As an antenna which is fixed to a window glass etc. of a vehicle, a film antenna has been known (see, e.g., Japanese Patent No. 3,294,837). This film antenna is of such a structure that a plurality pairs of antenna elements are formed on a film substrate and an amplifier having a connector is connected to one end of the film substrate. It described that the film substrate and the amplifier are fixed to a rear glass of a vehicle, and an earth line due to the element is formed on the film substrate, and at the side of the amplifier, there is no necessity to connect earth to ground. It also discloses that a plurality sets of film antennas and amplifiers are provided, and by switching them, diversity reception is carried out. In passing, it is possible to electrically switch antennas (see, e.g., JP-T-2002-510925).
Also known is such a structure that an antenna pattern is directly formed on a window glass of an automobile, and a booster amplifier is disposed on a glass surface (see, e.g., Japanese Patent No. 3175779). Also, known is such a structure of an in-vehicle flat antenna that, on at least a part of a window glass plate of a vehicle, a conductor layer for radiating element, a dielectric layer, and a grounding conductive layer are sequentially formed from underneath, and a weak signal which is transmitted from a GPS satellite is received (see, e.g., JP-A-5-63423). Further, known is such a structure that a hole is opened in a ceiling board of a vehicle body, and an antenna and an amplifier for receiving a weak signal from a GPS satellite are fixed thereto (see, e.g., JP-A-5-63419).
As an antenna for use in receiving radio waves which is fixed to a vehicle, preferred is a film antenna which is used by being stuck to a window glass. In this regard, however, as explained in a column of the problem that the invention of the patent document 1 is to solve, there is a necessity of connecting earth to a vehicle body, which requires a work which is difficult for a general user to do. In the patent document 1, it is described that, since an earth line is formed on a film substrate and can be connected to an amplifier by a connector, it is possible to solve a problem which comes up with connection of earth. However, there is no description as to a reason that connection to earth is possible by simply connecting through the connector to the earth line which is formed together with an element on the film substrate which is stuck on a glass surface.
In case that an amplifier is disposed in the vicinity of an antenna, as to earth connection of the amplifier, there is a necessity of carrying out DC earth including a direct current component, to a vehicle body. A vehicle body itself of an automobile is of a conductive material such as metal etc., but, normally, it is painted with a coating material which is an electric insulation material. In order to carry out DC earth to the suchlike vehicle body, as a process to the vehicle body, there is a necessity of opening a hole in the vehicle body, and of carrying out earth connection by use of a bolt etc.
Considering processing workability, it is conceivable that an amplifier is not disposed in the vicinity of an antenna. In this regard, however, in this case, a power supply coaxial cable from the antenna to the amplifier is elongated, and there occurs deterioration of a S/N ratio as a signal-to-noise ratio of a high-frequency reception signal due to cable loss, and reception quality is deteriorated. Also, when length of the coaxial cable becomes longer, in particular, when impedance of the coaxial cable is not matched with impedance at a power supply point of the antenna, noises which are generated from a vehicle are induced on the coaxial cable for power supply, and the noises from the vehicle are induced on the coaxial cable for power supply, and deterioration of reception quality is invited.